Electrochemical cell and cathode for same

ABSTRACT

A carbon cathode structure is described comprising an aggregate of discrete, porous, semi-rigid carbon conglomerates which are impressed from both sides onto a screen-like substrate for physical support thereby. The substrate additionally functions as a current collector by uniformly and electrically contacting the interior of the resulting structure. Electrolyte-conducting channels formed between adjacent conglomerates of the thick, porous cathode structure serve to maximize the cell rate capability and cell discharge capacity.

FIELD OF THE INVENTION

The present invention is related generally to electrochemical cells andis more particularly concerned with a novel construction for cellshaving high discharge capacities.

Cells with high discharge capacities are utilized for a wide variety ofapplications including portable lighting sources, communicationsequipment and the like. Since such cells may be required to supplycurrent over extended periods of time, it is desirable to provide thelargest discharge capacity per unit volume, hereinafter referred to asenergy density, and the highest possible rate capability.

One such electrochemical system characterized by a large energy densityof approximately 600 watt-hrs./kg. comprises an alkali metal anode whichis typically lithium but may also include sodium, potassium, etc., anelectrolyte containing an inorganic solvent selected from the groupconsisting of phosphorous oxychloride, thionyl chloride, sulfurylchloride and mixtures thereof, and a solute dissolved in the solvent.The cells additionally comprise a catalytic cathode material selectedfrom gold, carbon and (C₄ F_(N)) on which the solvent material iscatalytically reduced.

As used throughout the specification and claims, reference to aparticular anode or cathode material, shall mean the electrochemicallyactive component of the anode structure of the cathode material uponwhich the electrochemical reduction of the solvent takes place.

Owing to the unusually large volumetric energy densities of these cells,it is desirable to utilize them for such applications as those indicatedabove. However, as already stated, such applications require cellshaving a large discharge capacity and high rate capabilities in additionto large energy densities. Although it is well known in the art that therate capability of a cell may be increased by increasing the surfaceareas of the electrodes, since the electrochemical reactions within thecell occur where there is contact between electrolyte and theelectrodes, practical limitations exist with respect to the size whichthe cell may attain.

In the foregoing electrochemical system, the cathode has been found tobe the electrochemically weakest component during discharge of the cell.In other words, it is primarily responsible for a drop in cell voltageas the current draw is increased. An electrochemical cell having a highrate capability therefore requires a cathode which will sustain itspotential at high current densities.

In U.S. patent application Ser. No. 617,117, filed Sept. 26, 1975, nowabandoned assigned to the assignee of the present invention, andincorporated by reference, there is disclosed a porous, preformedthree-component cathode material and a method for making same. Thecathode material comprises 40 to 99% (by weight) of carbon black, atleast 1% of a binder, and graphite. Owing to the pores formed within thematerial, the electrolytic solution can penetrate the cathode surfaceand contact interior cathodic particles which thereby provide additionalactive surface areas to increase the rate capability of the cell, and toadditionally increase the cell discharge capacity without increasing thecell size. While, the cell discharge capacity may theoretically befurther increased by increasing the thickness of the cathodic material,substantial increases do not in fact occur beyond a critical cathodedimension. This limitation has been traced to the clogging of thecathodic pores with discharge products and the consequential blocking ofthe electrolytic solution from interior cathodic particles, therebydecreasing the active cathode surface area.

In U.S. patent application Ser. No. 614,467, filed Sept. 18, 1975,assigned to the assignee of the present invention and incorporated byreference, there is disclosed a cathode material comprising an aggregateof discrete porous semi-rigid globules. The porosity of each globuleresults from its composition and the process by which it is formed. Theindividual globules each have a multitude of minute pores which allowthe electrolytic solution to contact the interior cathodic particles andthereby provide a large active surface area over which the electrolytemay be chemically reduced. In addition to the pores contained within theindividual globules, larger fluid-conducting channels, defined betweenthe boundaries of adjacent globules, ensure that the electrolyticsolution can diffuse throughout the cathode material and contact theinterior cathodic particles to increase cell life. The resulitng cathodeis characterized by a high utilization which, consequently, increasesthe cell discharge capacity without an increase in cell size.

As taught in the above-referenced applications, cathodes utilizing thesematerials and adapted for use in small size electrochemical cells, suchas AA, C and D sizes are self-supporting preformed structures made byrolling or extruding a three-component carbon paste into its desiredshape. The wet cathodes are then dried and cured at elevatedtemperatures. These cathodes obtain their physical strength from linkageof binder particles and are generally self-supporting structures becauseof their small physical size.

Electrochemical cells having the foregoing electrochemical system haveconventionally assumed one of two general configurations. The first is abutton-type cell, such as that disclosed in U.S. Pat. No. 3,907,593,which is adapted for applications where low discharge currents arerequired for long periods of time. The second is a cylindrical cell suchas that shown in co-pending U.S. patent application Ser. No. 614,467,filed Sept. 18, 1975. Cylindrical cells, however, have inherent problemswhen one attempts to adapt that configuration to high discharge capacitycells having high rate capabilities. Thick electrodes such as thoseneeded for high rate cells are rigid and are not easily formed withinlarge cylindrical cells nor bent to follow the curvature of cylindricalcontainers.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrochemical cell witha high discharge capacity and having a novel cathode structure whichprovides a significantly higher rate capability discharge capacity thanheretofore achievable.

It is another object of the invention to provide a thick, porouselectrode cathode structure having a large, active surface area andrequiring a minimum of volume for inert current collectors.

ACcordingly, a high discharge capacity electrochemical cell is disclosedherein having thick, proous, prismatic cathodes which provide a large,active surface area and require a minimal volume for the inert currentcollecting materials. Porous self-supporting cathodes, such as thosedescribed in my above-reference applications, are structurally limitedby the mechanical strength of the binder. If the cell is to have adischarge capacity which requires physically large electrodes, thesecathode structures may crumble when handled during assembly of the cellor when subjected to physical shock while in the cell. The latter is ofparticular concern, since intra-cell shorting can occur.

The cathode structure of the instant cell is, therefore, aself-supporting structure comprising a rigid frame member, a metallicsubstrate having a plurality of interstice-defining members and affixedabout its periphery to the frame member, and an aggregate of discrete,semi-rigid of porous carbon conglomerates defining a network ofelectrolyte-conducting channels extending throughout the cathode. Theconglomerates are impressed on the interstice-defining substrate membersfor structural support thereby. The substrate members electricallycontact the cathode throughout its interior to function as a currentcollector for the cathode structure.

The conglomerates are formed by mixing a 40 to 99 weight percent ofcarbon black with at least 1 weight percent of a binder and a quantityof suitable liquid which is sufficient to form a slurry. The slurry isdried to form a semi-rigid, porous structure and broken into a mass ofcarbon conglomerates which are pressed onto both sides of a rigid,screen-like substrate to form the cathode structure.

These and other features of the invention will be more fully describedbelow in the following Description of the Preferred Embodiment which isto be read in conjunction with the Drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the Drawing:

FIG. 1 is a cross-sectional view of an electrochemical cell constructedin accordance with the invention; and

FIG. 2 is an isometric view of a cathode structure constructed inaccordance with the invention and utilized in the cell of FIG. 1; and

FIG. 3 is a side view in section of the cathode structure of FIG. 2.

Corresponding elements in the foregoing figures are identicallyreferenced.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning initially to FIG. 1, and electrochemical cell, shown generallyat 10, comprises an outer casing 12 and an electrochemical systemdisposed therein. The electrochemical system, in turn, comprises aplurality of alternately arranged anode and cathode structuresidentified as 14 and 16 respectively, and a thin, porous separators 18,made from an electrically, non-conductive material, disposed betweeneach adjacent anode and cathode to ensure electrical isolationtherebetween. The electrochemical system additionally comprises anelectrolyte 20 which is diffused throughout the cathode structure 14 andseparators 18.

The particular electrochemicl system may be selected from any known forwhich an inert conductive cathode collector material may be found. Anexample of such an electrochemical system is carbon-zinc system whichhas a zinc anode, a carbon cathode, and a electrolyte consisting of amoist, aqueous paste of amonium chloride, zinc chloride, manganese oxideand carbon particles. The system preferably utilized, however, belongsto a recently developed class of electrochemical cells where a lithium,or another alkaline metal, anode is utilized with an electrolyticsolution which includes a liquid oxyhalide solvent material and a solutedissolved therein to make the solution ionically conductive. Suitableoxyhalide solvents include those of phosphorous or sulfur, such asphosphorous oxychloride, thionyl chloride, sulfuryl chloride, ormixtures thereof. Such oxyhalide solvent materials additionally functionas liquid depolarizers as they are electrochemically reduced on thesurface of the cathode material during operation of the cell. Suitableelectrochemical systems are set forth in co-pending application Ser. No.685,214, filed May 11, 1976 and U.S. Pat. No. 3,923,543.

The anode structure 16 depicted in FIG. 1 is particularly suited for theinstant cell and forms the subject matter of a U.S. patent applicationentitled "Prismatic Anode for an Electrochemical Cell with HighDischarge Capacity", Ser. No. 763,848 filed concurrently herewith, andhereby incorporated by reference.

The cathode structure 14 depicted in FIGS. 2 and 3 provides a relativelythick structure having the requisite active surface area for a high ratecapability cell. The structure 14 comprises an electrically conductivesubstrate 32 having a plurality of interstice-defining members. Thesubstrate may conveniently be a screen, a sheet of expanded metal, orpreferably a sheet of distorted expanded metal. The latter, known by theacronym DISTEX, comprises expanded metal having interstice-definingmembers which are twisted 90° along each interstice for added structuralstrength. In the preferred embodiment, the substrate 32 is made fromnickel.

Affixed to the periphery of the substrate 32 is a rigid, metallic frame30. In the preferred embodiment, the substrate is spot welded about itsentire periphery to the leg 30a of a "T"-shaped electrically conductiveframe as will be more evident below. The resulting structure provides ahighly porous cathode current collector which structurally reinforcesthe cathodic material without inhibiting the diffusion of theelectrolyte throughout the cell.

The cathode material 34, generally comprised from a graphite, from about40 to 99 weight percent of carbon black, and at least 1 weight percentof a binder which is inert in the cell and which is preferably afluorocarbon polymer such as is sold under the trademark Teflon byDupont.

The graphite and carbon black utilized in the formation of the cathodematerial are preferably of commercial grade or better purity. Thegraphite particle size is preferably maintained below 650 mesh and thecarbon black utilized is preferably compressed about 50%. Thesepreferred specifications for the graphite and carbon black are selectedto ensure a homogeneous product which will not contribute to adeterioration of the discharge parameters of the cell through theincorporation of reactive impurities therein.

Two examples of the preferred fluorocarbon polymers of particularutility in the present invention, are those identified by the trademarksTeflon and Kel-F. Teflon is a registered trademark of E. I. DuPont deNemours and Company for tetrofluoroethylene fluorocarbon polymers andfluorinated ethylene-propylene resins. Kel-F is a registered trademarkof the 3M Company for a series of fluorocarbon products includingpolymers of chlorotrifluoroethylene and certain co-polymers. Thefunction of fluorocarbon polymer is to stabilize the mechanical strengthof the cathode material by forming chain-like connections between thevarious particles of graphite and carbon black to form a mechanicalbinding network so that a semi-rigid configuration may be achieved forthe cathode material.

The particular composition of the cathode material which is preferredcomprises up to 30 weight percent of graphite, 65 to 99 weight percentof carbon black and 1 to 10 percent of fluorocarbon polymer. Theparticular composition chosen results in cathode material having varyingporosity characteristics. The variance in porosity is beneficial becauseit permits a concomitant variance in the discharge rates available fromthe resulting primary electrochemical cell.

In the preferred embodiment, the cathodic material 34, is prepared as adough preferbly by dry mixing 350 grams of carbon black withapproximately 35 grams of graphite for about 15 hours. Approximately 1.5liters of a 50% isopropyl solution in water is added to the dry mix tocarbon graphite and mixed for approximately 2 additional hours to form aslurry. An amount of binder equal to approximately 5% of the weight ofthe dry mix is mixed into the slurry until uniformly dispersedtherethrough. The resulting dough is extruded into a spaghetti-like formapproximately 5 millimeters in diameter and dried for approximately 15hours at room temperature. Although other methods of shaping the doughmay be used, extrusion generaly precludes any smearing of the doughsurface which adversely effect porosity. The spaghetti is chopped toform a plurality of conglomerates having a length of from 0.1 to 5millimeters, depending on the size of the cathode. Generally thickercathodes require larger fluid conducting channels between globules toenable the electrolyte to reach the interior portions of the cathodestructure. These larger channels are provided by the use of largerconglomerates. It has been found, by way of example, that a cathodestructure such as that illustrated by the numeral 14 having dimensionsof 12 inches × 21 inches × 1/2 inches performed well with conglomerateshaving a length of 10 millimeters.

The conglomerates thus formed are impressed onto the substrate 32 fromboth sides to form a thick, coarse layer of cathodic material coveringboth sides of the substrate, and the interior periphery of the frame 30.The pores formed within each conglomerate, and the network offluid-conducting channels defined between adjacent conglomerates,ensures the complete diffusion of electrolyte throughout the thickcathode structure 14 throughout the discharge of the electrochemicalcell. It may also be appreciated that the substrate 32, being embeddedin the cathodic material and contacting the interior thereof, provides auniform current distribution over the total cathodic surface areawithout affecting electrolyte diffusion therethrough.

The net result of the above-described cathode structure, is a large,thick, highly active cathode having good conductivity and materialstrength. The electrode porosity enhances the rate capability of thecell, while its large physical size and the network of channels maximizethe cell discharge capacity.

The cathode structure 14 thus formed, is electrically connected to acell terminal 40 by means such as a conductive bracket 36 affixed to theouter periphery of the frame 30 and mechanically coupled to acurrent-collecting cross-bar 38. The terminal 40 is actually the top endof a cylindrical rod which passes through a compression seal 44 in thecover 42 of the cell casing. As is known in the art, compression sealscomprise a metallic barrel 48, a rod such as the terminal 40 and a layer49 of electrically non-conductive insulating material such as afluorethylene polymer therebetween which electrically insulates the rodfrom the barrel 48. The barrel 48 is welded to the casing cover 42 tohermetically seal the electrochemical system within the casing 12 whenthe cover 42 is welded about its periphery thereto. The lower end of therod 40 forms a lug 50 which may be connected to the current collectorcross-bar 38. The cell described herein utilizes parallel electrodeswhich result in the most uniform current distribution over the electrodesurface during discharge. Cells having dimensions of 5 × 32 × 40 cm havebeen found to have a volumetric energy density of 950 W-H/liter, and adischarge capacity of 1900 A-H at a current level of 5-10A and anaverage cell voltage of 3.3 volts. The rate capability of the cell isdemonstrated by its short-circuit current of 800A. Other cells have beenconstructed having discharge capacities of 12,000A-H and short-circuitcurrents of 4,000A.

While there have been shown and described what are considered to bepreferred embodiments of the present invention, it will be obvious tothose of ordinary skill in the art that various changes andmodifications may be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

We claim:
 1. A self-supporting cathode structure for use in anelectrochemical cell comprising:a rigid, electrically conductive,T-shaped frame member; an electrically conductive generally planarsubstrate having a plurality of interstice-defining members and affixedabout its periphery to the leg of the T-shaped frame member; and anaggregate of discrete, porous carbon conglomerates impressed on theinterstice-defining members of the substrate from opposite sides of thesubstrate and being in mutual physical and electrical contact throughthe interstices, a network of electrolyte-conducting channels extendingthroughout the resulting porous carbon structure; theinterstice-defining member of the substrate electrically contacting theinterior of the porous carbon structure throughout the porous carbonstructure to function as a current collector for the cathode.
 2. Thecathode structure of claim 1 wherein the substrate is an expanded metalscreen.
 3. The cathode structure of claim 2 wherein the porous carbonstructure comprises an aggregate of discrete porous globules definingthe electrolyte-conducting channels therebetween.
 4. The cathodestructure of claim 3 wherein the globules are in the order of 0.1 to15mm in length.
 5. An electrochemical cell comprising:a case; first andsecond terminals located on the exterior of the case; means forelectrically insulating the second terminal from the first terminal; ananode structure within the case and electrically connected to the firstterminal; a self-supporting cathode structure formed from:a rigid,electrically conductive, T-shaped frame member electrically connected tothe second terminal, an electrically conductive generally planarsubstrate having a plurality of interstice-defining members and affixedabout its periphery to the leg of the T-shaped frame member, and anaggregate of discrete, porous carbon conglomerates impressed on theinterstice-defining members of the substrate from opposite sides of thesubstrate and being in mutual physical and electrical contact throughthe interstices, a network of electrolyte-conducting channels extendingthroughout the resulting porous carbon structure, theinterstice-defining members of the substrate electrically contacting theinterior of the porous carbon structure throughout the porous carbonstructure to function as a current collector for the cathode; anelectrolytic solution including a cathode depolarizer that iselectrochemically reduced on the porous carbon structure; and a porousseparator of non-conductive material disposed between the anode andcathode structure.